Hitherto, coating of a metal substrate with a synthetic resin has been practiced in an attempt to improve corrosion resistance of the metal substrate. For example, a synthetic resin-clad lining pipe wherein an inner surface of the metal pipe is clad with a synthetic resin is manufactured through the diameter-expansion method or the diameter-reduction method.
According to the diameter-expansion method, a metallic pipe and a synthetic resin pipe for cladding are provided in advance and the following processes are performed: an adhesive layer is formed on the inner surface of the metallic pipe; a synthetic resin pipe for cladding is placed inside the metallic pipe; and the synthetic resin pipe for cladding is expanded by the application of heat and pressure, to thereby laminate the synthetic resin layer onto the metallic pipe via the adhesive. This method is disclosed in Japanese Patent Application Laid-Open (kokai) Nos. 56-55227, 58-12720, and 59-59418.
According to the diameter-reduction method, an adhesive layer is provided on the inner surface of a metal pipe, and a synthetic resin pipe placed inside the metallic pipe is press-laminated against the metal pipe by reduction of the diameter of the metal pipe achieved by use of rolls, etc.
With regard to adhesives which may be used in manufacturing such synthetic resin-clad lining pipes, Japanese Patent Application Laid-Open (kokai) No. 56-55227 discloses a polyamide-based or polyester-based adhesive, and Japanese Patent Application Laid-Open (kokai) No. 58-12720 discloses a butadiene-styrene block copolymer elastomer-based hot-melt adhesive. There have also been used other adhesives containing polyolefin which has been copolymerized with an unsaturated aliphatic carboxylic acid or an anhydride thereof.
However, adhesion durability between a metal substrate and a synthetic resin for cladding is not satisfactory even when the above-mentioned adhesives are used for lamination of a synthetic resin on a metal substrate. Since the coefficient of linear expansion of a metal substrate and that of a synthetic resin for cladding significantly differ from each other, a metal/synthetic resin laminate easily permits delamination of the synthetic resin for cladding from the metallic pipe by repeated cooling-heating cycles. Similarly, delamination also occurs after repeated expansion and contraction due to changes in atmospheric temperature. Also, when adhesion between metal and synthetic resin is insufficient, delamination tends to be caused due to application of stress--such as bending, blanking or cutting--to the synthetic-resin-laminated metal. Thus, improvement of adhesion between a metal substrate and a synthetic resin to be laminated is demanded.